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Abstract:

Techniques herein prepare an alloy catalyst using a protective conductive
polymer coating. More particularly, an alloy catalyst is prepared by:
preparing a platinum catalyst supported on carbon; coating the surface of
the platinum catalyst with a conductive polymer; supporting a transition
metal salt on the coated catalyst; and heat treating the catalyst on
which the transition metal salt is supported. Also, an alloy catalyst may
be prepared by: preparing a platinum-transition metal catalyst supported
on carbon; coating the surface of the platinum-transition metal catalyst
with a conductive polymer; and heat treating the coated catalyst.
Accordingly an alloy catalyst with superior dispersity can be prepared by
increasing the degree of alloying of the catalyst through heat treatment
while preventing the increase of catalyst particle size through
carbonization of the conductive polymer. The prepared catalyst may be
useful, for example, for a fuel cell electrode.

Claims:

1. A method for preparing an alloy catalyst comprising: preparing a
platinum catalyst supported on carbon; coating the surface of the
platinum catalyst with a conductive polymer; supporting a transition
metal salt on the coated catalyst; and heat treating the catalyst on
which the transition metal salt is supported.

2. The method for preparing an alloy catalyst according to claim 1,
wherein the carbon is one or more selected from a group consisting of:
carbon black, carbon nanotube, carbon nanofiber, carbon nanocoil, and
carbon nanocage.

3. The method for preparing an alloy catalyst according to claim 1,
wherein the conductive polymer is one of either polypyrrole or
polyaniline.

4. The method for preparing an alloy catalyst according to claim 1,
wherein the transition metal is selected from a group consisting of:
cobalt, iron, nickel, palladium, ruthenium, titanium, vanadium, and
chromium.

5. The method for preparing an alloy catalyst according to claim 1,
wherein the transition metal salt is one or more selected from a group
consisting of: nitrate, sulfate, acetate, chloride and oxide comprising
cobalt, iron, nickel, palladium, ruthenium, titanium, vanadium, and
chromium.

6. The method for preparing an alloy catalyst according to claim 1,
wherein the heat treatment is performed at a temperature within a range
of 700-1000.degree. C.

7. The method for preparing an alloy catalyst according to claim 1,
wherein the alloy catalyst is configured for use as a fuel cell
electrode.

8. A method for preparing an alloy catalyst comprising: preparing a
platinum-transition metal catalyst supported on carbon; coating the
surface of the platinum-transition metal catalyst with a conductive
polymer; and heat treating the coated catalyst.

9. The method for preparing an alloy catalyst according to claim 8,
wherein the carbon is one or more selected from a group consisting of:
carbon black, carbon nanotube, carbon nanofiber, carbon nanocoil, and
carbon nanocage.

10. The method for preparing an alloy catalyst according to claim 8,
wherein the conductive polymer is one of either polypyrrole or
polyaniline.

11. The method for preparing an alloy catalyst according to claim 8,
wherein the transition metal is selected from a group consisting of:
cobalt, iron, nickel, palladium, ruthenium, titanium, vanadium, and
chromium.

12. The method for preparing an alloy catalyst according to claim 8,
wherein the heat treatment is performed at a temperature within a range
of 700-1000.degree. C.

13. The method for preparing an alloy catalyst according to claim 8,
wherein the alloy catalyst is configured for use as a fuel cell
electrode.

14. A system for preparing an alloy catalyst comprising: means for
preparing a platinum catalyst supported on carbon; means for coating the
surface of the platinum catalyst with a conductive polymer; means for
supporting a transition metal salt on the coated catalyst; and means for
heat treating the catalyst on which the transition metal salt is
supported.

15. A system for preparing an alloy catalyst comprising: means for
preparing a platinum-transition metal catalyst supported on carbon; means
for coating the surface of the platinum-transition metal catalyst with a
conductive polymer; and means for heat treating the coated catalyst.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. §119 to
Korean Patent Application No. 10-2010-0119155, filed on Nov. 26, 2010, in
the Korean Intellectual Property Office, the disclosure of which is
incorporated herein by reference in its entirety.

BACKGROUND

[0002] (a) Technical Field

[0003] The present invention relates to a method for preparing a
high-activity alloy catalyst for a fuel cell having a large active area
by coating a carbon-supported platinum or platinum-transition metal
catalyst with a conductive polymer such as polypyrrole (PPy) as a capping
agent and performing heat treatment, thus increasing the degree of
alloying and catalytic activity while preventing growth of particle size.

[0004] (b) Background Art

[0005] Fuel cells which convert chemical energy resulting from oxidation
of fuel directly into electrical energy are spotlighted as a
next-generation energy source. In particular, research for
commercialization has been actively carried out in the auto industry for
the purposes of improvement of fuel efficiency, reduction of emissions,
environmental protection, or the like. In particular, extensive research
has been focused on the catalysts for oxidation and reduction occurring
on the electrodes of the fuel cells.

[0006] For commercialization of the polymer electrolyte membrane fuel cell
(PEMFC), performance, cost, and durability issues have to be solved, all
of which are closely related to the fuel cell catalyst. Since the PEMFC
operates at low temperature, a precious metal such as platinum is used as
a catalyst in order to increase the slow rate of oxygen reduction.
However, the high cost and limited reserves of platinum delay its
commercialization. For a fuel cell vehicle to be commercially viable, it
is reported that the use of platinum should be reduced below 0.2 g per
kW. However, theoretically, voltage loss occurs as the supporting amount
of platinum decreases (e.g., to 0.4 mg/cm2 or lower). Thus, there is
a limitation on reducing the use of platinum in the platinum catalyst.
Further, since the slow oxygen reduction leads to over-voltage in the
cathode, alloy catalysts are studied to improve the reaction rate.

[0007] As an alloy catalyst for a fuel cell, Pt3M of an oriented
face-centered cubic structure with M being a transition metal (e.g., Ti,
V, Cr, Fe, Co, Ni, etc.) is studied actively. Carbon-supported Pt3M
is typically prepared by depositing a metal precursor on a commercially
available carbon-supported platinum catalyst and carrying out heat
treatment. Usually, after adding a transition metal precursor to the
carbon-supported platinum catalyst, heat treatment is carried out at
700-1200° C. using a gaseous reducing agent such as hydrogen.
Although the heat treatment improves catalytic activity by increasing the
degree of alloying, it is accompanied by increased particle size and
decreased dispersity.

[0008] Accordingly, methods for preparing alloy catalysts without the heat
treatment process are studied. For example, certain previous methods to
prepare alloy catalysts below 200° C. used carbonyl complexes
(e.g., as described by Hui et al. in J. Phys. Chem. 108 (2004)
11024-11034), while other previous methods used the microemulsion method
(e.g., as described by Xiong et al. in Electrochim. Acta 50 (2005)
2323-2329). However, the general colloid method is difficult in control
of the degree of alloying and the transition metal easily dissolves out
under the fuel cell environment since its concentration on the surface is
high. One additional method, such as that described by Watanabe et al. in
Appl. Mater. Interfaces 2 (2010) 888-895, succeeded in preparing small
alloy particles with desired composition by using nanocapsules. One
drawback to this method, however, is that it is not easy to remove the
oleic acid and oleylamine used to create the capsules and, as a result,
they lead to reduced catalytic activity. As such, since the currently
available methods for preparing alloy catalysts at low temperatures have
many problems, heat treatment at high temperature is unavoidable.

SUMMARY

[0009] The present invention coats a conductive polymer such as
polypyrrole (PPy) or polyaniline (PANI) on a platinum or
platinum-transition metal catalyst supported on carbon in order to
prevent growth of catalyst particles and then performed heat treatment at
high temperature. As a result, carbonization of the conductive polymer
during the heat treatment inhibited growth of the metal catalyst
particles and led to formation of face-centered cubic alloy of platinum
and the transition metal, resulting in increased platinum concentration
on the catalyst surface and reduced dissolving out of the transition
metal. Accordingly, the present invention is directed to providing a
method for preparing an alloy catalyst with increased degree of alloying
and restricted particle growth, by introducing a conductive polymer such
as PPy or PANI as a capping agent and carrying out heat treatment at high
temperature.

[0010] In one general aspect, the present invention provides a method for
preparing an alloy catalyst including: preparing a platinum catalyst
supported on carbon; coating the surface of the platinum catalyst with a
conductive polymer; supporting a transition metal salt on the coated
catalyst; and heat treating the catalyst on which the transition metal
salt is supported.

[0011] In another general aspect, the present invention provides a method
for preparing an alloy catalyst including: preparing a
platinum-transition metal catalyst supported on carbon; coating the
surface of the platinum-transition metal catalyst with a conductive
polymer; and heat treating the coated catalyst.

[0012] The above and other aspects and features of the present invention
will be described further herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above and other objects, features and advantages of the present
invention will now be described in detail with reference to certain
exemplary embodiments thereof illustrated in the accompanying drawings
which are given hereinbelow by way of illustration only, and thus are not
limitative of the disclosure, and wherein:

[0014] FIG. 1 schematically illustrates a procedure of preparing an alloy
catalyst by coating polypyrrole (PPy) on a carbon-supported platinum
catalyst as a capping agent and then depositing a precursor;

[0015]FIG. 2 schematically illustrates a procedure of preparing an alloy
catalyst by coating PPy on a carbon-supported platinum-transition metal
catalyst as a capping agent wherein NaBH4 reduction is used;

[0016] FIG. 3 shows molecular structures of PPy and polyaniline (PANI)
that can be used as capping agent;

[0021] FIG. 8 compares current-voltage (CV) characteristics of
platinum-cobalt alloy catalysts (Pt3Co1/CNC) supported on CNC
prepared by heat treatment with or without coating with PPy and a
commercially available carbon-supported platinum-cobalt alloy catalyst;
and

[0022] FIG. 9 compares oxygen performance of platinum-cobalt alloy
catalysts (Pt3Co1/CNC) supported on CNC prepared by heat
treatment with or without coating with PPy and a commercially available
carbon-supported platinum-cobalt alloy catalyst.

[0023] It should be understood that the appended drawings are not
necessarily to scale, presenting a somewhat simplified representation of
various preferred features illustrative of the basic principles of the
disclosure. The specific design features of the disclosure as disclosed
herein, including, for example, specific dimensions, orientations,
locations and shapes, will be determined in part by the particular
intended application and use environment.

DETAILED DESCRIPTION

[0024] Hereinafter, reference will now be made in detail to various
embodiments of the present invention, examples of which are illustrated
in the accompanying drawings and described below. While the disclosure
will be described in conjunction with exemplary embodiments, it will be
understood that the present description is not intended to limit the
disclosure to those exemplary embodiments. On the contrary, the
disclosure is intended to cover not only the exemplary embodiments, but
also various alternatives, modifications, equivalents and other
embodiments, which may be included within the spirit and scope of the
disclosure as defined by the appended claims.

[0025] The present invention relates to preparation of an alloy catalyst
by coating the surface of a platinum or platinum-transition metal
catalyst supported on carbon with a conductive polymer and carrying out
heat treatment, thus increasing the degree of alloying, preventing growth
of catalyst particle size through carbonization of the conductive
polymer, and providing superior dispersity.

[0026] The platinum alloy catalyst may be prepared in two ways. Platinum
and a transition metal may be reduced simultaneously and then heat
treated. Alternatively, platinum may be reduced first and, after
impregnating a transition metal salt later, it may be reduced by heat
treatment (precursor deposition). When metals of several species need to
be reduced at once, a very strong reducing agent is required. NaBH4
is usually used. However, use of such a strong reducing agent is
problematic in that the control of metal particle size is difficult, the
degree of alloying decreases due to difference in reducing rate of the
metals, and the concentration of the transition metal on the catalyst
surface tends to be high. Due to low equilibrium potential, the
transition metals on the surface are easily dissolved under the fuel cell
operation environment, leading to decreased fuel cell performance. If the
degree of alloying of the alloy catalyst is increased, the concentration
of the transition metal on the catalyst surface decreases, resulting in
improved durability as well as catalytic activity. In order to increase
the degree of alloying of the alloy catalyst, a heat treatment process
has to be accompanied. However, heat treatment at high temperature leads
to decreased active area due to increased particle size. Thus, in the
present invention, a conductive polymer is introduced as a capping agent
in order to increase the degree of alloying of the catalyst by heat
treatment while preventing particle size increase.

[0027] For example, Korean Patent No. 10-0728611 proposes a catalyst
prepared by coating a conductive polymer on a carbon support,
distributing platinum and a transition metal on the coated film and
carrying out heat treatment. That is to say, in the above patent, the
conductive polymer is coated on the support and then the active
substances are introduced on the conductive polymer. In contrast, in the
present invention, a platinum or platinum-transition metal catalyst
supported on a catalyst is coated with a conductive polymer and then heat
treatment is carried out. As a result, the conductive polymer is
carbonized, thus preventing the growth of catalyst particles enclosed
therein while allowing the formation of the face-centered cubic structure
of platinum and the transition metal. Accordingly, the present invention
is quite different from the above patent in the technological background,
preparation process, role of the conductive polymer, and so forth.

[0028] The present invention provides a method for preparing an alloy
catalyst comprising: preparing a platinum catalyst supported on carbon;
coating the surface of the platinum catalyst with a conductive polymer;
supporting a transition metal salt on the coated catalyst; and heat
treating the catalyst on which the transition metal salt is supported.

[0029] First, as shown in FIG. 1, a platinum catalyst supported on carbon
is prepared. The method of supporting of platinum on carbon is not
particularly limited. Such known methods as chemical reduction using a
reducing agent, alcohol reduction, polyol method, or the like may be
used. The carbon support may be one or more selected from commonly used
carbon black, carbon nanotube, carbon nanofiber, carbon nanocoil and
carbon nanocage.

[0030] Then, the surface of the platinum catalyst is coated with a
conductive polymer. The conductive polymer may be polypyrrole (PPy) or
polyaniline (PANI), the structures of which are shown in FIG. 3. The
conductive polymer may be coated by immersing the platinum catalyst in an
organic solvent such as ethanol, adding a monomer of the conductive
polymer thereto, and polymerizing the monomer at 4° C. in the
presence of an oxidizing agent. Specifically, the monomer of the
conductive polymer may be pyrrole. After the coating is completed, the
catalyst having the surface coated with the conductive polymer may be
obtained after washing and drying.

[0031] Then, a transition metal salt is supported on the coated catalyst.
The transition metal may be cobalt, iron, nickel, palladium, ruthenium,
titanium, vanadium or chromium, and the transition metal salt may be one
or more selected from nitrate, sulfate, acetate, chloride and oxide of
the metals. After adding the transition metal salt together with the
coated catalyst to an organic solvent such as ethanol and refluxing at
80° C., the organic solvent may be evaporated to obtain the
catalyst with the transition metal salt supported.

[0032] Finally, the catalyst with the transition metal salt supported is
heat treated to obtain an alloy catalyst. The heat treatment is performed
at 700-1000° C. under argon or argon/hydrogen mixture atmosphere.
If the heat treatment temperature is below 700° C., the degree of
alloying of the catalyst may be low, leading to unsatisfactory activity.
And, if the temperature exceeds 1000° C., although the degree of
alloying increases, the catalytic activity may decrease due to increased
particle size. Hence, the aforesaid range is preferred.

[0033] If necessary, the heat-treated catalyst may be refluxed in an
aqueous solution of a strong acid to remove impurities.

[0034] The present invention further provides a method for preparing an
alloy catalyst comprising, as shown in FIG. 2: preparing a
platinum-transition metal catalyst supported on carbon; coating the
surface of the platinum-transition metal catalyst with a conductive
polymer; and heat treating the coated catalyst.

[0035] Alternatively from the above-described method, platinum and a
transition metal are supported on carbon first, and heat treatment is
carried out after coating with a conductive polymer. In general, a
stronger reducing agent is needed to support platinum and the transition
metal at once on carbon by reducing them and the control of particle size
is difficult. However, if the platinum-transition metal catalyst can be
prepared with small and uniform particle size, by coating with the
conductive polymer and then carrying out the heat treatment, the degree
of alloying can be improved while preventing the increase of catalyst
particle size. Details about the transition metal, the conductive polymer
and the heat treatment condition may be the same as those of the
above-described method, wherein the platinum catalyst is coated with the
conductive polymer and then the transition metal salt is supported.

[0036] According to the method for preparing an alloy catalyst of the
present invention, an alloy catalyst with superior dispersity can be
prepared by increasing the degree of alloying of the catalyst while
preventing the increase of catalyst particle size. The prepared catalyst
may be useful, for example, for a fuel cell electrode.

EXAMPLES

[0037] Illustrative examples and experiments will now be described. The
following examples and experiments are for illustrative purposes only and
not intended to limit the scope of this disclosure.

Example 1

Preparation of Alloy Catalyst Using Platinum Catalyst Supported on CNF

[0038] 1-Pyrenecarboxylic acid (1-PCA, 250 mg) was stirred in ethanol (400
mL) for 30 minutes. Herringbone-type carbon nanofiber (CNF, 500 mg) was
added to the 1-PCA solution and stirred for 6 hours. Then, CNF doped with
1-PCA was recovered through filtration under reduced pressure. 1-PCA was
introduced to CNF in order to form π-π interaction between pyrene
and graphene of CNF. The 1-PCA-doped CNF has a hydrophilic surface and
allows easy supporting of platinum. The 1-PCA-doped CNF (140 mg) was
added to ethylene glycol (25 mL) and stirred for 10 minutes. Then, after
adding NaOH (100 mg), the mixture was further stirred for 10 minutes. The
concentration of NaOH was 0.1 M. NaOH serves to adjust pH for control of
the platinum particle size. Subsequently, after adding PtCl4 (150
mg) and stirring for about 10 minutes, the mixture was refluxed at
160° C. for 3 hours. Then, platinum ions are reduced and adsorbed
on the surface of CNF. In order to further increase the supporting ratio,
after stirring at room temperature for 12 hours, the solution was further
stirred for 24 hours after lowering pH to 1-2. The resulting solution was
filtered under reduced pressure and washed several times with ultrapure
water. The recovered catalyst was dried at 160° C. for 1 hour to
remove impurities.

[0039] The prepared catalyst was stirred in ethanol (20 mL) and then
stirred at 4° C. for 1 hour after adding pyrrole (130 mg).
Thereafter, an aqueous solution (17.6 mL) of the oxidizing agent ammonium
persulfate (228 mg) prepared by dissolving in water (100 mL) was added
and stirring was carried out at 4° C. for 24 hours. Then, pyrrole
is polymerized to polypyrrole (PPy). After completion of the
polymerization, the resulting catalyst was recovered by filtration under
reduced pressure, washed sufficiently with water and ethanol, and dried
in a vacuum oven at 40° C. for 12 hours. A PPy-coated platinum
catalyst supported on CNF was obtained.

[0040] Co(NO3)2.6H2O (43 mg) and ethylenediamine (40.4 mg)
were sufficiently stirred in ethanol. Then, the coated catalyst prepared
above was added to the solution and refluxed at 80° C. for 3
hours. Upon completion of the refluxing, the solvent was evaporated using
an evaporator to support the transition metal salt on the catalyst.

[0041] The catalyst with the transition metal salt supported was heat
treated in a furnace at 800° C. for 1 hour, under argon 90 vol
%/hydrogen 10 vol % atmosphere. Upon completion of the heat treatment,
the catalyst was recovered by cooling to room temperature. Then, after
refluxing in 0.5 M sulfuric acid at 80° C. for 3 hours in order to
remove impurities, followed by washing and drying, an alloy catalyst was
obtained.

Example 2

Preparation of Alloy Catalyst Using Platinum Catalyst Supported on CNC

[0042] An alloy catalyst was prepared in a manner similar to Example 1,
except for using carbon nanocage (CNC) instead of CNF and carrying out
heat treatment at 900° C.

Example 3

Preparation of Alloy Catalyst Using Platinum-Cobalt Catalyst supported on
CNF

[0043] A platinum-cobalt catalyst was prepared using CNF as a support. CNF
(130 mg) was stirred in ultrapure water (25 mL) for 10 minutes. Then,
after adding PtCl4 (150 mg) and CoCl2 (19.3 mg), the mixture
was stirred for 10 minutes. An aqueous solution of NaBH4 prepared by
dissolving NaBH4 (224.7 mg) in ultrapure water (25 mL) was used as a
reducing agent. Upon completion of reduction, the platinum-cobalt
catalyst supported on CNF was recovered by filtration under reduced
pressure, washed several times with ultrapure water, and dried at
160° C. for 1 hour.

[0044] The prepared platinum-cobalt catalyst was coated with PPy and heat
treated in the same manner as Example 1 to obtain an alloy catalyst.

Comparative Example 1

Preparation of Alloy Catalyst Using Platinum Catalyst supported on CNF

[0045] An alloy catalyst was prepared in the same manner as Example 1,
without coating with PPy.

Comparative Example 2

Preparation of Alloy Catalyst Using Platinum Catalyst supported on CNC

[0046] An alloy catalyst was prepared in the same manner as Example 2,
without coating with PPy.

Comparative Example 3

Preparation of Alloy Catalyst Using Platinum-Cobalt Catalyst Supported on
CNF

[0047] An alloy catalyst was prepared in the same manner as Example 3,
without coating with PPy.

[0050] When a strong reducing agent is used to prepare a platinum alloy
catalyst, the control of metal particle size is difficult and the
concentration of the transition metal on the catalyst surface increases
due to the difference in reducing rate of the metals. The transition
metal on the surface is easily dissolved under the fuel cell operation
environment, leading to decreased fuel cell performance. Accordingly,
heat treatment at high temperature has to be accompanied to decrease the
concentration of the transition metal on the catalyst surface. However,
the heat treatment at high temperature leads to decreased active area due
to increased particle size. Therefore, introduction of a conductive
polymer as a capping agent is necessary to increase the degree of
alloying of the catalyst by heat treatment while preventing particle size
increase.

[0051] Platinum particle size was calculated for the Pt (111) peaks at
2θ=40° using the Scherrer formula. The Pt3Co1/C
catalyst prepared by NaBH4 reduction at room temperature had a
particle size of 3.2 nm, which increased to 21.8 nm when the catalyst was
heat treated at 800° C. (Comparative Example 3). The degree of
alloying was determined from the shift of the Pt (111) peaks. More
rightward shift relative to the XRD peak of the general carbon-supported
platinum catalyst is translated into a higher degree of alloying. The
Pt3Co1/C catalyst prepared by NaBH4 reduction at room
temperature showed a rightward 2θ shift of 0.9° as compared
to the general Pt/C catalyst, and the Pt3Co1/C catalyst
prepared by NaBH4 reduction followed by heat treatment at
800° C. (Comparative Example 3) exhibited a rightward 2θ
shift of 1.2°. This reveals that the Pt3Co1/C catalyst
prepared by NaBH4 reduction at room temperature has a low degree of
alloying and requires heat treatment. If the degree of alloying is low,
the concentration of the transition metal on the surface increases.
Although the degree of alloying is increased by heat treatment, the
particle size is also increased. The increase of the catalyst particle
size results in decreased active area of the catalyst. When the
Pt3Co1/C catalyst was prepared by precursor deposition, i.e.,
by reducing platinum on carbon, supporting the transition metal precursor
and then carrying out heat treatment (Comparative Example 1), the
particle size was 8.5 nm. When the Pt3Co1/C catalyst was
prepared by precursor deposition using PPy as the capping agent (Example
1), the particle size was 3.5 nm. The catalyst prepared by precursor
deposition showed a rightward 2θ shift of 1.3°, and the
catalyst prepared by precursor deposition using PPy as the capping agent
showed a rightward 2θ shift of 1.4°. That is to say, the
catalyst of Example 1 had a smaller particle size and a higher degree of
alloying than the catalyst of Comparative Example 1. Thus, it was
confirmed that precursor deposition using PPy as the capping agent can
increase the degree of alloying while preventing the increase of particle
size.

[0052] X-ray diffraction patterns of the carbon-supported platinum-cobalt
alloy (Pt3Co1/C) catalysts prepared by NaBH4 reduction
followed by heat treatment using PPy as the capping agent (Example 3) or
without using PPy (Comparative Example 3) are shown in FIG. 5.

[0053] The Pt3Co1/C catalyst prepared using PPy as the capping
agent (Example 3) had a particle size of 3.7 nm, whereas the
Pt3Co1/C catalyst prepared without using PPy (Comparative
Example 3) had a particle size of 21.8 nm. This confirms that use of PPy
as the capping agent in the preparation of the alloy catalyst is also
useful in NaBH4 reduction as well as in precursor deposition.

[0054] Inductively Coupled Plasma (ICP) Analysis

[0055] ICP analysis was carried out for the carbon-supported
platinum-cobalt alloy (Pt3Co1/C) catalysts prepared by
NaBH4 reduction at room temperature, NaBH4 reduction followed
by heat treatment at 800° C. (Comparative Example 3), precursor
deposition (Comparative Example 1), and precursor deposition using PPy as
a capping agent (Example 1). The result is summarized in Table 2.

[0056] When NaBH4 reduction was carried out at room temperature, the
Pt:Co ratio was 3.2:1, with higher Pt than the desired Pt:Co ratio of
3:1, as cobalt dissolved out on the catalyst surface during acid
treatment. In contrast, in the other three catalysts which were heat
treated, the dissolution of Co into the acid solution was less than the
NaBH4 reduction at room temperature. This result suggests that the
proportion of platinum increased on the catalyst surface whereas that of
cobalt increased at the catalyst core during the heat treatment.
Considering that a fuel cell is operated under an acidic condition, heat
treatment is necessarily required to increase the proportion of platinum
on the surface of the alloy catalyst. Accordingly, use of PPy as the
capping agent in precursor deposition is effective as the strategy to
increase the proportion of platinum on the surface of the alloy catalyst
while preventing the increase of particle size.

[0059] Platinum particle size was 2 nm in (a), which increased to 3-12 nm
in (d) after heat treatment. It can also be seen that the dispersity of
platinum became worse. When PPy was used as the capping agent, it can be
seen that PPy was coated on Pt/CNF with a thickness of 2 nm in (b). It
can be seen in (c) that the platinum-cobalt alloy maintained a small
particle size of 3.5 nm and high dispersity after the heat treatment at
800° C.

[0061] Platinum particle size was 3.7 nm in (a), which increased to 5-15
nm in (d) after heat treatment. It can also be seen that the dispersity
of platinum became worse. When PPy was used as the capping agent, it can
be seen that PPy was coated on Pt/CNC with a thickness of 3 nm in (b). It
can be seen in (c) that the platinum-cobalt alloy maintained a small
particle size of 5.0 nm and high dispersity.

[0063] Also, unit cell oxygen performance was tested for the catalysts.
The anode was prepared with a commercially available Pt/C catalyst, at
0.4 mg/cm2 on the basis of platinum, and the cathode was prepared
with each catalyst, at 0.4 mg/cm2 on the basis of the metal.
Hydrogen (150 ccm) and oxygen (150 ccm) were supplied to the anode and
the cathode, respectively, under normal pressure. The unit cell was
operated at 75° C. and performance was evaluated by current
density measured at 0.6 V. The result is shown in FIGS. 8-9 and Table 3.

[0064] The active surface area of the catalyst measured by the CV test was
41.5 m2/g for the Pt3Co1/CNC catalyst prepared using PPy
as the capping agent (Example 2), larger than 20.8 m2/g of the
Pt3Co1/CNC catalyst prepared by normal heat treatment
(Comparative Example 2) or 31.8 m2/g of the commercially available
Pt3Co1/C catalyst. This suggests that use of PPy as the capping
agent increased the degree of alloying through heat treatment at high
temperature while preventing the increase of particle size and
maintaining uniform dispersity.

[0065] In the unit cell oxygen performance test, the Pt3Co1/CNC
catalyst prepared using PPy as the capping agent (Example 2) showed a
current density of 1.83 A/cm2 at 0.6 V, whereas the
Pt3Co1/CNC catalyst prepared by normal heat treatment
(Comparative Example 2) exhibited 1.20 A/cm2. The commercially
available Pt3Co1/C catalyst showed a current density of 1.71
A/cm2 at 0.6 V. This result coincides with the active surface area
measurement by CV. The use of PPy as the capping agent increased the
degree of alloying through heat treatment at high temperature while
preventing the increase of particle size, and thus increased the
catalytic activity.

[0066] According to the method for preparing an alloy catalyst of the
present invention, an alloy catalyst with superior dispersity can be
prepared by increasing the degree of alloying of the catalyst through
heat treatment while preventing the increase of catalyst particle size
through carbonization of the conductive polymer. The prepared catalyst
may be useful, for example, for a fuel cell electrode.

[0067] The present invention has been described in detail with reference
to specific embodiments thereof. However, it will be appreciated by those
skilled in the art that various changes and modifications may be made in
these embodiments without departing from the principles and spirit of the
disclosure, the scope of which is defined in the appended claims and
their equivalents.